1. Field of the Invention
This invention generally relates to quench systems for cooling hot flue gases from combustion systems. More specifically, the invention relates to a method and apparatus to cool hot flue gases from thermal oxidation of chemical wastes to their adiabatic saturation temperature.
2. Description of the Related Art
Flue gases from thermal oxidation processes frequently require cooling to their adiabatic saturation temperature to enable them to be further processed (i.e., in a packed column absorber, venturi scrubber, etc.), before they are discharged to the atmosphere. Hot flue gases from thermal oxidation systems have temperatures typically in the range of 1400 to 2000 degrees F. The adiabatic saturation temperature is the minimum gas temperature which can be achieved using water evaporation when there is no other heat loss from the system. The adiabatic saturation temperature will vary with the system pressure, water content of the hot flue gas, and other factors, but is largely a function of the temperature of the incoming hot flue gases. When hot flue gas temperatures are in the range of 1400 to 2000 degrees F., the adiabatic saturation temperatures are in the range of approximately 150 to 195 degrees F.
Many hot flue gas quench systems achieve adiabatic cooling of hot flue gases by using spray nozzles in a duct or spray contactor vessel. Typically, numerous nozzles are connected to the water source, and the nozzles take a pressure drop of approximately 80-100 psi to produce fine water drops which promote rapid cooling of the hot flue gases passing through the duct. Spray nozzle cooling systems have several significant disadvantages including substantial valving and piping requirements, and the tendency of the nozzles to corrode or erode and to plug up with solid particles which are frequently present in the hot and usually corrosive recycled spray water. In addition, a high pressure pump is needed to supply the nozzles with water at the required pressure.
Venturi scrubbers have been used for removing pollutants, either solids or gaseous, from gas streams, by contacting the gas stream with a finely divided liquid, i.e., water droplets. Solid particles or gases are absorbed by the water droplets which are subsequently coalesced and the liquid containing the solids or gases removed. A venturi scrubber accelerates the gas stream to a high velocity, and the throat may be adjustable, if desired, to permit variation of the pressure drop during operation. The pressure drop is important because it determines the size of water droplets which largely determines the venturi's performance. For example, Sly Manufacturing and Andersen 2000 Inc. market venturi scrubbers having adjustable throats. At the venturi throat, where the gas stream is accelerated to a high velocity, the gas stream comes into contact with a scrubbing liquid that is introduced. The primary purpose of a scrubber is maximum contact of the gas and fluid streams with the most effective use of the required pressure drop. Downstream from the venturi throat, the particulate-laden droplets collide, agglomerate in a vessel or storage tank, and may be collected for disposal.
The venturi scrubbers described above also have been used for cooling hot flue gases. For example, some venturi scrubbers are mounted in duct work. Venturi scrubbers often include a separator downstream of the venturi for removing the water droplets from the gas stream.
Flow rates of hot flue gases from thermal oxidation vary considerably during operation. To accommodate these changes and to achieve the adiabatic saturation temperature for the exhaust gases at different flow rates, devices have been used for adjusting the throat of the venturi so that the pressure drop may be changed. Some devices for adjusting the venturi throat have disadvantages including difficulty of maintaining seals against high pressure, erosion due to high velocities in the venturi throat, corrosion problems associated with acidic or basic liquids flowing through the ducts, and space requirements for the apparatus used for adjusting the throat.
Another method of handling varying flow rates through a venturi is to size the venturi for a high pressure drop at maximum flow conditions which would still allow for a sufficient pressure drop at low flow rates. However, this would be a very energy intensive means of operation because of the combustion air fan power requirements for such a venturi.
Another alternative for cooling hot flue gases is a submerged quench system marketed by T-Thermal Company which bubbles hot gases through a pipe submerged in water to achieve adiabatic cooling of hot flue gases. A disadvantage of a submerged quench system is the potential for turbulent wave action which could cause high mechanical stress to the equipment, and wave action in the water tank which could result in pressure pulses in the system.